Project Abstract Although the stem cell microenvironment, or the niche, has been extensively studied as a site which regulates stem cell functions, immunological attributes of the niche have been largely unexplored. Interestingly, studies conducted over six decades ago revealed that the locations of germline and embryonic stem cells, the testis and placenta, immunological sanctuaries for stem cells, termed immune privileged (IP) sites. In these tissues, transplanted allogeneic or xenogeneic grafts can persist without immune suppressive therapy, distinguishing IP sites from other sites. Little is known about whether tissue-committed stem cell niches serve as IP sites. Our recent studies demonstrated evidence suggesting that the HSC niche within the bone marrow (BM) is an IP site. We showed that unique FoxP3+ regulatory T cells (Tregs) with high expression of an HSC marker, CD150, frequently localized adjacent to HSCs. These Tregs enabled persistence of allo-HSCs in non- conditioned immune competent mice without immune suppression. In non-transplantation settings, niche Tregs protected endogenous HSCs from oxidative stress, maintaining HSC quiescence. These observations suggest that niche Tregs endow HSCs with immune privilege. To promote translation of our findings into clinical settings, we will propose to study roles and therapeutic potential of niche Tregs in hematopoiesis failure following irradiation and in allo-HSC engraftment following non-myeloablative conditioning. We will characterize niche Tregs' stress-resistance, repopulating potential, molecular signatures, and T cell receptor (TCR) repertoire, assessing how these features of niche Tregs contribute to HSC protection or engraftment. Successful studies will identify unprecedented Treg subsets which act as stem cell protectants from immune attack and stress within the niche, decipher mechanisms of this niche Treg-mediated stem cell protection or engraftment, and lead to development of promising therapeutic strategies for post-irradiation BM injury or engraftment by transferring or manipulating niche Tregs. Therefore, the results of our proposed studies will eventually improve outcomes of allo-BM transplantation or radiation therapy for patients with blood disorders, immune deficiency, metabolic diseases, blood malignancies, and solid cancers.